1. Field of the Invention
The present invention relates to kilns, to nozzles for kilns, and to waste slurries, more particularly, to methods of burning waste slurries in cement kilns, and to waste slurries, kilns and kiln nozzles for use in such methods. The present invention is especially useful in connection with waste slurries containing hazardous waste.
2. Description of the Prior Art
Many types of solid combustible waste generated by industry and elsewhere are categorized as hazardous waste because of, e.g., their flammable or toxic characteristics. Recent environmental regulations have imposed strict restrictions on disposal of hazardous waste, frequently making it necessary to employ expensive high-temperature incinerators with extentive emission control devices.
Cement kilns have received favorable review from federal and state environmental regulatory agencies as providing ideal conditions for disposal of combustible waste materials. Not only does the burning of hazardous waste in operating kilns allow for recovery of energy values from hazardous waste, but also, because of their high operating temperatures, long residence time and ability to provide favorable conditions for chemical combination of inorganic residues into the active components of cement, such operations provide excellent conditions for environmentally sound disposal of combustible hazardous waste.
The burning of hazardous waste, however, faces a number of practical limitations. Before the promulgation of existing EPA air quality regulations, it was a practice to charge combustible solid waste into the cold end of cement kilns with the mineral materials being processed into cement. With current regulations, however, it would be impossible to follow such practices. Combustible hazardous waste contains many volatile organic substances, which would result in unacceptable hydrocarbon emissions because the volatile components are driven off into effluent gases at temperatures below those required for thermal degradation of the volatilized components.
One attempt to provide a system in which hazardous waste can be burned in a cement kiln is disclosed in U.S. Pat. No. 4,974,529. In accordance with the method disclosed in that patent, hazardous waste is containerized and fed into a kiln through a port formed in the kiln wall, the port being aligned with a drop tube inside the kiln. Hazardous waste fuel is delivered to the kiln through the port at predetermined times during operation of the kiln. This method is inconvenient and complicates the design of kilns, and it requires introduction of the fuel in uneven pulses.
U.S. Pat. No. 4,806,056 discloses an apparatus and a method for using materials such as coarse pieces of pneumatic tire casings as a secondary source of heat energy in applications such as lime kilns, cement kilns, boiler furnaces and the like. Hazardous waste sludges can be metered to mix and coat the surfaces of tire-derived fuel so that the hazardous materials can be fed into a cement kiln. The patent notes that in order to dispose of some hazardous wastes safely, it is necessary to expose them to sufficiently high temperatures for long enough periods of time. According to the patent, for delivery of tire-derived fuel for use in cement kilns and lime kilns, it is preferred to deliver the fuel pneumatically through a large pipe, the size of which must be adequate to permit passage of the pieces of tire-derived fuel without being clogged. Referring to FIG. 6, air is fed through an exit mouth 64 to a fuel delivery conduit 54, taking along with it comminuted tire-derived fuel 48 delivered through an infeed conduit 56. In one preferred application, shown in FIG. 8, the stream of air and tire-derived fuel proceeds through nozzle 70 and is discharged into the combustion chamber of a lime kiln 72.
U.S. Pat. No. 4,976,210 discloses a method and apparatus for treating hazardous waste materials in which bulk material such as heavy solid hazardous waste material is supplied via a hopper to a hydraulic ram 28 (see FIG. 1) or is supplied via a flow line and pumped into the intake 18 of the kiln.